1. Field of the Invention
The present invention relates generally to an image sensor used to transform optical signals into electrical signals. More particularly, the invention relates to a complementary metal oxide semiconductor (CMOS) image sensor and a method of fabricating the same.
2. Description of the Related Art
Complementary metal-oxide semiconductor (CMOS) image sensors (CISs) and charge coupled device (CCD) image sensors are commonly employed in a variety of modern technologies including, for example, cellular phones, personal digital assistants (PDAs), digital cameras. CMOS and CCD image sensors transform light incident on a array of two-dimensional photodiodes into electrical charges, which are subsequently transformed into electrical voltages after a predetermined exposure period. One major difference between CMOS image sensors and a CCD image sensors, however, is the manner in which the electrical charges corresponding to each photodiode are transformed into electrical voltages. In a CCD image sensor, the electrical charges are sequentially transferred to a common output structure where they are transformed into electrical voltages. In a CMOS image sensor, on the other hand, each photodiode has its own individual output structure for transforming the electrical charges into electrical voltages.
A number of problems may occur in conventional CMOS image sensors due to the presence of dark current or other forms of noise. For example, dark current is known to deteriorate charge transmission efficiency and charge storage capacity, which can lead to image defects such as pixel saturation. The phrase “dark current” refers to electrical charges that are accumulated at a photodiode in the absence of incident light. It is reported that dark current is usually caused by silicon dangling bonds existing on the surface of a silicon substrate. At relatively high thermal ranges, these dangling silicon bonds generate negative charges that can be accumulated by the photodiode even in the absence of incident light. Thus, if several silicon dangling bonds exist on the surface of the silicon substrate, the image sensor may behave as if light were present, even if the image sensor is in a dark place.
Several technologies have been proposed to reduce dark current in image sensors (see U.S. Pat. No. 6,730,899, No. 6,714,241, and No. 6,649,950). However, these methods are limited to reducing dark current only within specific portions of a unit pixel.
For example, in a photodiode of a typical CMOS image sensor, various interface defects may exist near a semiconductor substrate surface in a P+-type hole accumulated device (HAD) region formed on an N-type photodiode. Such defects result from fabrication processes performed on the semiconductor substrate, silicon dangling bonds, or the like. Charges generated by the interface defects are introduced into an N-type photodiode through various passages, and travel to a floating diffusion region through a transfer gate region along with electrons generated by incident light on the photodiode. As a result, these charges show up as outputs from a source follower buffer amplifier. The output voltage resulting from the defects is added to the output voltage caused by incident light, thereby causing image defects such as white spots on images.
Because of the problems caused by dark current, there is a general demand in the industry for image sensors that exhibit reduced dark current, such as the dark current caused by dangling silicon bonds on a semiconductor substrate surface.